TerraTrike Zoomer, stretched 18" to acomodate engine. Lifan engine with internal CVT running to a sproket on a left-hand freewheel (so the engine and the pedals are totally independent). Rear wheel custom built by The Wheelmaster. Speed - cruises at 30 -40mph all day long, even with a passenger and loaded trailer (I rode it from Seattle to Ocean Park WA a few years ago). Cost - I spent about $3,000 on it, but could do it again for about $2,000.

Amazing! What a thoroughly practical trike I really like that, - in fact I wouldn't mind having a go at gathering together several old bicycles from under the house and seeing if I could build something similar

That is excellent! I thought stretching frame length would affect steering more, but people here have done the same thing as you (on an old sociable tandem trike frame). It is almost 9 feet long and a small gas motor was bolted on.

I built mine from 2004 to 2005; but it has a plain very shrouded factory trike frame with the fairing lowered and stretched and a hub motor.

If 30 is the goal, piece of cake, like Alaskavan said, stretch a proven design (look at atomiczombie's designs) like the one in the pic.
drive one side of the wheel with the pedals, and the other with a 50cc lifan

If the budget were, say 3k (4 tops for the first one) one could easily do REALLY nice trike, with fiberglass body and canopy.

The Bicycleman.com sells electric motor kits and a wide variety of recumbents. Fairings by Zzipper or Muller are available to fit recumbents and upright bikes. These are starting points off the shelf. Another possiblity is to stretch the frame on a 50cc ATV and add a cab. The Blue Sky racer is a great chassis but still a little low slung for traffic.